Reliable and rapid data communication is important in terrain and aerospace vehicles to ensure accurate reporting of various vehicle conditions. For instance, in order to properly and safely operate an aircraft, a pilot (or remote operator) is typically provided with updates on the operational status of the personnel, passengers, physical integrity, or mechanical moving parts of the aircraft. Numerous update types are possible; these are but a few of the many types of tangible and operational data points that can be important.
Communication can be difficult in these vehicles because of harsh environmental conditions and difficulty in routing communication channels. In addition, there is a tradeoff between the desire to monitor data points and the resulting increased vehicle weight and space requirements. The cabling necessary to monitor and transmit data from the far reaches of the vehicle to the operator adds weight and bulk to the vehicle. The more data monitored the more weight and bulk, which reduces the vehicle's performance and increases its fuel consumption. In addition, as the data may result in operational changes that must be transmitted back to the data source (or a related area), two way communication is often necessary, further increasing cabling.
These and other matters have presented challenges to communication circuitry and channels for a variety of applications.
As one example, electrical signals maybe interrupted by electromagnetic interference (EMI) or radio frequency interference (RFI). EMI and RFI present challenges to communication circuits and channels. This problem is exacerbated by the plurality of cables in modern vehicles.
EMI is a disturbance that affects an electrical circuit due to electromagnetic induction or electromagnetic radiation from an external source. The effect of the disturbance can range from completely blocking the electrical signal to limiting or degrading the signal. The source of EMI may be either natural such as solar flares from the sun or meteor showers in the atmosphere, or artificial such as localized magnetic fields, adjacent electrical circuits or conductors, power transmission lines, or radio signals. Narrowband EMI is intended radiation from transmissions such as radio or television systems, or cell phones. Broadband EMI is unintentional radiation from sources such as electric power transmission lines. RFI is EMI at high (radio) frequency.
Susceptibility is a measure of how badly a piece of equipment is affected in the presence of EMI. Electromagnetic Pulses (EMP) occur when the disturbance is of short duration or a series of short duration pulses. Examples of this are electrostatic discharge, nuclear electromagnetic pulse, or pulse trains from gasoline ignition systems.
There are four basic coupling mechanisms for EMI. They include conductive, capacitive, magnetic or inductive, and radiated. Conductive coupling happens through direct contact like through a transmission line and can be common-mode (noise in same phase on both conductors) or differential-mode (noise is out of phase on both conductors). Capacitive coupling occurs when a varying electric field exists between two adjacent conductors typically less than a wavelength apart, inducing a change in voltage across the gap. Magnetic or inductive coupling occurs when a varying magnetic field exists between two parallel conductors typically less than a wavelength apart, inducing a change in voltage along the receiving conductor. Radiated or electromagnetic coupling occurs when there is a large distance between a transmitter and a receiver (more than a wavelength) and the transmitter radiates an electromagnetic wave which is picked up by the receiver.
In looking for solutions to EMI, the problem must be characterized into understanding the source and signal creating the interference, the coupling path to the receiver, and the electrical nature of the receiver and the significance of the interference to the function of the receiver. EMI cannot always be completely eliminated, but can be reduced to an acceptable level of interference with a given probability.
Radio reception may be affected by EMI. Analog circuits are particularly affected by EMI. Digital circuits are more immune to noise as they can use error correction techniques to eliminate some or all unwanted EMI interference.
The common methods of reducing electromagnetic interference in devices and metal cored electrical conductor cables have several issues. All require extra components which add size, weight and complexity to the conductors and overall system.